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    <title>Document</title>
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<body>

</body>
<script>
    function success(text) {   
        var textarea = document.getElementById('test-response-text');
        textarea.value = text;
    }

    function fail(code) {
        var textarea = document.getElementById('test-response-text');
        textarea.value = 'Error code: ' + code;
    }

    var request = new XMLHttpRequest(); // 新建XMLHttpRequest对象

    request.onreadystatechange = function () { // 状态发生变化时，函数被回调
        if (request.readyState === 4) { // 成功完成
            // 判断响应结果:
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                // 成功，通过responseText拿到响应的文本:
                return success(request.responseText);
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                // 失败，根据响应码判断失败原因:
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    // 发送请求:
    request.open('GET', '/api/categories');
    request.send();

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